Oklahoma Cooperative Extension Service EPP-7646

Diseases of in Oklahoma January 2017 John Damicone Extension Plant Pathologist Oklahoma Cooperative Extension Fact Sheets are also available on our website at: Lynn Brandenberger http://osufacts.okstate.edu Extension Horticulturist

Introduction Asparagus is a well-adapted vegetable for Oklahoma and can be a valuable commercial crop in the state. Asparagus is perennial and a planting may remain productive for more than 15 years. Spear size and yield will decrease after 12 years, but a planting should produce 2,500 pounds of spears per acre. More will be produced each year during the most productive period of the planting. Site selection and soil preparation for new plantings are discussed in Extension Fact Sheet HLA-6018, Asparagus Production. Yield is dependent on the amount of nutrients (carbohydrates) stored in the root system and plant density in a field. Carbohydrates are produced by the previous season’s fern growth and move into the roots as ferns age. Therefore, maintenance of vigorous growth from harvest until fern senescence in the fall is essential for maximizing yield. Poor vigor can be caused by several stress factors that damage ferns or weaken crowns. These include an excessively long cutting period (four weeks the first year of cutting and Figure 1. Fern defoliation and premature death caused by eight weeks thereafter is recommended), insufficient water, Cercospora blight. Purple spot causes similar damage. weed competition, inadequate soil fertility, poorly drained soil, soil compaction, defoliation from larvae of the asparagus beetle and diseases. This Fact Sheet is intended to provide ferns may be blighted by late July in a wet year (Figure 1). asparagus producers with information necessary to identify The disease results in reduced photosynthesis of affected and manage asparagus diseases of potential importance in ferns. Yield loss the following spring is a result of reduced Oklahoma. crown vigor caused by the early defoliation. Repeated yearly defoliation by Cercospora blight weakens crowns and reduces stand productivity and longevity. First-year asparagus is rarely Cercospora blight (Cercospora asparagi) affected by Cercospora blight. Cercospora blight is an important foliar disease of as- Control: An integrated approach is recommended for paragus in Oklahoma. It can be particularly severe in the management of Cercospora blight. Implementation of several eastern half of Oklahoma where rainfall and humidity levels cultural practices will help provide partial control. Schedule favor infection. The over-winters on fern residues left overhead irrigations to allow thorough drying of the foliage on the soil. Spores produced on this debris infect ferns after before nightfall, or use drip or furrow irrigation to keep foliage harvest, when conditions are favorable for infection. Favorable dry. In eastern Oklahoma, utilization of a wider row spacing conditions include relative humidity of 95 percent or higher of 6 feet, rather than the usual 5, will increase air movement and average daily temperatures of 77 to 86 F. Symptoms first to dry foliage and delay canopy closure. Rows should also appear on lower portions of the ferns after row closure and be planted in a north-south direction to take advantage of periods of high humidity. Symptoms are small, oval, grey to prevailing southerly winds in drying foliage. tan lesions (spots) with reddish brown borders on the needles Research has shown that burning, but not tillage, of and small branches. Spores of the fungus are produced on the infested residue in the spring delays blight appearance by lesions and are dispersed by wind and rain. Development of about one week. However, asparagus residue left on the the disease generally depends on rainfall and humidity levels soil is beneficial for soil conservation, reducing abrasion of during June to August. Browning progresses upward from the emerging spears by wind-blown sand and for maintaining lower ferns as conditions favor spread of the disease. Entire organic matter content of soil.

Division of Agricultural Sciences and Natural Resources • Oklahoma State University Foliar applications of a fungicide recommended for control of Cercospora blight provide excellent control. In comparison to untreated plots, yield increases of 2,000 pounds per acre have been achieved in research plots in the harvest season following treatment the previous year with fungicide sprays. Applications should be made at three-week intervals begin- ning at row closure when the ferns are approximately 4 feet in height. Sprays should be applied in a sufficient volume of water to provide thorough coverage of the foliage. There are no known asparagus varieties resistant to Cercospora blight. Consult the latest edition of the E-832, Extension Agents Handbook of Insect, Plant Disease and Weed Control for the latest list of fungicides recommended for Cercospora blight control.

Figure 3. Purple spot on an asparagus fern. Purple spot (Stemphylium vesicarium/ Pleospora herbarum) Purple spot is another damaging foliar disease of as- October. Blighting of foliage before natural senescence in the paragus in Oklahoma caused by a fungus that has sexual fall reduces the translocation of carbohydrates into the crowns. (Pleospora) and asexual (Stemphylium) forms. Losses from As a result, yield in the year following severe foliar blighting this disease are from spotting of spears, which reduces their can be reduced by 50 percent. Residue management reduc- marketability and blighting of ferns after harvest, which reduces es disease development, but is not practical because tillage crown productivity the following spring. The fungus overwinters typically damages crowns, predisposing them to Fusarium on asparagus residue left on the soil where the fungus produces crown and root rot. A fungicide program where applications tiny black fruiting bodies that produce airborne spores formed are begun in late July and applied on 14- to 21-day intervals as a result of sexual recombination. Spores released from the is effective in controlling purple spot. infested residue initiate the disease during spring. Symptoms appear on spears as lesions (spots) that have tan to gray Stem, Crown, and Root Rot - Decline centers and dark purplish borders (Figure 2). Purple spot on spears can be severe during cool, wet springs. Twelve hours (Fusarium spp.) of wetness is necessary for infection and the disease is more Stem, crown and root rot are caused by pathogenic severe with abrasion from wind-blown sand. On expanded strains and species of a common soilborne fungus whose ferns after harvest, tan- to brown-colored lesions occur on populations increase in soil cropped to asparagus. Symptoms branches and needles. Lesions are typically surrounded by of these diseases are seedling blight and stunting, yellowing a purple border (Figure 3) and appear similar to Cercospora and death of plants in older plantings. Yellowing usually first blight. The presence of the black fruiting bodies of Pleospora, appears in midsummer and on one or two ferns per plant. that appear as tiny black specks visible with the eye or a 10X The bright yellow color of affected plants is in sharp contrast hand lens are characteristic of purple spot. Spots elongate, to remaining green and healthy plants (Figure 4). Uprooting merge together, and cause extensive foliage blight (Figure 1). affected plants reveals a dark, reddish-brown decay of lower The disease is typically most severe from late August through stems, crowns, and roots (Figure 5). Reddish-brown internal

Figure 2. Purple spot on asparagus spears (photo D. Johnson, Washington State University). Figure 4. Yellowing of fern caused by Fusarium stem, crown, and root rot.

EPP-7646-2 Figure 6. Brick-red pustules on a fern branch with . Figure 5. Fusarium stem and crown rot on a one-year old asparagus crown. discoloration of crowns is evident when affected crowns are after cutting has been terminated. Rust may also develop on split with a knife. These crowns eventually decay completely ferns allowed to develop during cutting. Rust appears first and die. as raised, green spots (pustules) on stems and branches of “Decline” occurs when plant density in the field is reduced developing ferns. Spores from these pustules reinfect ferns and vigor of remaining plants is diminished to the extent that and produce brick-red pustules on stems and branches (Figure maintenance of the stand is no longer profitable. When an 6). A different spore type from these brick-red pustules also attempt to replant affected sites is made, stand establishment reinfects ferns and results in increased rust pressure. Rust is difficult and spear productivity is low. The severity of decline is easily identified at this stage by observing the rust-colored is increased by low soil pH, both excessive and insufficient soil powdery mass of spores released from these pustules when moisture, high nitrogen levels and excessive cutting periods. rubbed against white paper or light-colored clothing. These The fungi that cause stem, crown and root rot persist in red pustules turn black in color as the spots age. The black soil indefinitely. Diseased crowns and contaminated asparagus color is a result of the formation of dark-colored resting spores seeds are thought to be a means of introducing this disease which over-winter and are capable of initiating the disease into clean fields. the following year. Conditions favoring rust development are Control: Control of this disease is extremely difficult once periods of prolonged heavy dew formation and dry soil. it becomes established in a field. Therefore, measures should Damage caused by rust is a result of reduced crown vigor be taken to exclude it from fields. This is primarily done by when plants are infected several years in succession. Severe planting healthy, vigorous crowns. For direct seeding, soak infection over several years will greatly weaken crowns to the seeds in diluted chlorine bleach (1 pound of seed per gallon extent that yield is reduced. of a 1-part bleach/8-part water dilution) for 40 minutes, dry and Control: The best control measure is to plant rust-resistant apply a fungicide seed treatment before planting. Consider varieties. Most of the newer hybrid varieties are rust resistant. establishing beds with hybrid varieties that have resistance If a susceptible variety is used, rows should be spaced six feet to Fusarium rather than with susceptible open-pollinated apart and oriented in a north-south direction to maximize air varieties. New fields should have good surface and internal movement and drying down the rows. During harvest, cut all water drainage and adequate nutrition provided before crowns spears, fern growth and non-harvested plants to ground level are planted. Soil pH should be maintained between 6.5 and or below. This will aid in breaking the disease cycle and may 7.5. Recommended pest, fertility and irrigation management halt rust development. If rust appears on ferns after harvest, practices should be implemented to keep ferns actively the fungicide spray schedule recommended for management growing throughout the season and to minimize stress. If the of Cercospora blight will also provide rust control. disease becomes serious, the field should be rotated away from asparagus. Re-establishment of asparagus should not be attempted in fields with a history of decline problems. Reference Elmer, W.H. 2001. The economically important diseases of Rust ( asparagi) asparagus in the U.S. Online. Plant Health Progress doi:10.1094/PHP-2001-0521-01-RV. Asparagus rust occurs to varying degrees wherever asparagus is grown. The fungus attacks developing ferns

EPP-7646-3 The Oklahoma Cooperative Extension Service WE ARE OKLAHOMA The Cooperative Extension Service is the largest, • It provides practical, problem-oriented education most successful informal educational organization for people of all ages. It is designated to take in the world. It is a nationwide system funded and the knowledge of the university to those persons guided by a partnership of federal, state, and local who do not or cannot participate in the formal governments that delivers information to help people classroom instruction of the university. help themselves through the land-grant university • It utilizes research from university, government, system. and other sources to help people make their own Extension carries out programs in the broad catego- decisions. ries of agriculture, natural resources and environment; • More than a million volunteers help multiply the family and consumer sciences; 4-H and other youth; impact of the Extension professional staff. and community resource development. Extension staff members live and work among the people they • It dispenses no funds to the public. serve to help stimulate and educate Americans to • It is not a regulatory agency, but it does inform plan ahead and cope with their problems. people of regulations and of their options in Some characteristics of the Cooperative Extension meeting them. system are: • Local programs are developed and carried out in • The federal, state, and local governments co- full recognition of national problems and goals. operatively share in its financial support and • The Extension staff educates people through program direction. personal contacts, meetings, demonstrations, • It is administered by the land-grant university as and the mass media. designated by the state legislature through an • Extension has the built-in flexibility to adjust its Extension director. programs and subject matter to meet new needs. • Extension programs are nonpolitical, objective, Activities shift from year to year as citizen groups and research-based information. and Extension workers close to the problems advise changes.

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